You can sell it for whatever you like.
You've bought it from them, purified at cost, sold on.
Like any other product in the market.
Not sure you'll make millions from it though bud.
My mrs uses about 3 or 4 litres a month.
There's 4 of us in our house.
On your figures, you're looking at £2.50 a month.
Better off selling in bulk to other shiners it cleaning yourself.
Fella here was selling @ £10 for 650l a few years back.
He had 6 lads all filling almost daily.
He made some $ on that!
Despite claiming to be a 'church mouse'
Fair play to him I say.
To you, perhaps your margins might be higher selling in larger quantities or commercially.
All the best though.